The decision to reinstate the officer follows a lengthy inquiry by Greater Manchester Police and the Police Complaints Authority.

A dossier was compiled including interviews with more than 300 people and passed to the Crown Prosecution Service, which decided not to press charges last year.

But an inquest into Mr Kernan's death is not expected to be held until September.

Mrs Kernan said: "This just goes to prove the police can do what they like. Their job is supposed to be policing but they think they can act as judge and jury as well.""

She added: "I will not let this rest, somebody has to be held accountable."

Mr Makin said: "I find it quite incomprehensible that, three years this July from the date of this gross tragedy, here we have a person who admitted shooting the deceased, returning to duty before the inquest has even concluded.

"It may well be that there has been no prosecution but that may change when the inquest actually resumes."

He added: "Mrs Kernan's claim for compensation really is just a bagatelle. She is interested in justice. She wants her day when she can criticise the mental health services and the police."

A spokeswoman for Merseyside police said: "The force would not want to pre-judge the coroner's inquest into the death of Mr Kernan.

"However, after lengthy and thorough investigations carried out by independent sources the force does not expect there to be any criticism of the officer's conduct."

Since her son's death Mrs Kernan has called for police to be given better training in how to cope with people with mental illness.